//对象转查询字符串
function objToSearchString(obj) {
    var str = ``
    for (var key in obj) {
        str += `${key}=${obj[key]}&`
    }
    return str.slice(0, str.length - 1)
}
//封装ajax函数
function ajax(method, url, params, cb) {
    //1.创建请求对象
    var xhr = new XMLHttpRequest()
    //2.配置请求对象
    xhr.open(method, method == 'get' ? url + "?" + objToSearchString(params) : url)
    //3.设置回调函数
    xhr.onreadystatechange=function(){
        if(xhr.readyState==4){
            cb(JSON.parse(xhr.responseText))
        }
    }
    //4.发送请求
    //先对post请求设置 content-ype请求头
    xhr.setRequestHeader('content-type','application/x-www-form-urlencoded')
    xhr.send(method=='get'?null:objToSearchString(params))
}